The use of recombinant DNA technology has allowed the engineering of host cells to produce desired compounds, such as polypeptides and secondary metabolites. The large scale production of polypeptides in engineered cells allows for the production of proteins with pharmaceutical uses and enzymes with industrial uses. Secondary metabolites are products derived from nature that have long been known for their biological and medicinal importance. Because of the structural complexity inherent in such molecules, traditional chemical synthesis often requires extensive effort and the use of expensive precursors and cofactors to prepare the compound. In recent years, the expression of heterologous proteins in cells has facilitated the engineering of heterologous biosynthetic pathways in microorganisms to produce metabolites from inexpensive starting materials. In this manner, a variety of compounds have been produced, including polyketides, β-lactam antibiotics, monoterpenes, steroids, and aromatics.